Certain lamps are known to include a discharge tube to facilitate the illumination function. For example, U.S. Pat. No. 6,137,229 discloses a conventional metal halide lamp with a ceramic discharge tube. As shown in U.S. Pat. No. 6,137,229, end portions of conventional discharge tubes are known to comprise ring portions with a wall thickness based on the power supplied to the lamp.
FIGS. 1-3 depict a further example of a conventional ceramic discharge tube 160. As shown, the discharge tube 160 includes end portions 164a, 164b disposed on opposite circumferential end portions of a substantially cylindrical tubular member 162. The discharge tube 160 is symmetrically disposed about an elongated axis 158 and includes an outer radius “r” of 9.35 millimeters. Each end portion 164a, 164b is substantially identical and includes a transition section 168 connected between a tubular extension 166 and the body portion. Each end portion further includes a ring portion 173 connected between the transition section and the body portion. As shown in FIG. 3, the transition section 168 includes an exterior radius “r1” of 2 millimeters and an interior radius “r2” of 0.81 millimeters wherein the ratio r1/r2 is 2.46. The ring portion includes a thickness “t1” of 1.5 millimeters and the end portion includes an outer radius “r3” of 8.55 millimeters wherein the ratio t1/r3 is 0.176. It is also known to provide an end portion with a ratio r1/r2 of 2.46 and a ratio t1/r3 of 0.23.
As shown in FIG. 2, the transition section 168 spans between a maximum extent 168a in the direction of the elongated axis 158 and a minimum extent 168b in the direction of the elongated axis 158. The minimum extent 168b has a first dimension “d1” of 1.5 millimeters with respect to an interior surface 172. The maximum extent 168a has a second dimension “d2” of 3.4 millimeters with respect to the interior surface 172.
Conventional end portions can have features that result in cracking due to heat-cycles during the lamp lifetime. There is a continued need to provide discharge tubes with features that inhibit cracking of one or more end portions of discharge tubes.